Stihl MS 250 Won’t Start (5 Insider Fixes for Quick Revive)

The crisp air bites with a promise of winter, and the familiar scent of woodsmoke hangs heavy in the evenings. For many of us, that means it’s time to get the chainsaw roaring and stockpile firewood. But what happens when your trusty Stihl MS 250, the workhorse of countless wood-cutting projects, decides to stay silent? Believe me, I’ve been there. There’s nothing more frustrating than wrestling with a stubborn saw when the daylight is fading and the woodpile is dwindling.

Over the years, I’ve spent countless hours felling trees, bucking logs, and troubleshooting my own equipment. I’ve learned a thing or two about these machines, and I’m here to share some insider fixes to get your Stihl MS 250 back in action. We’re diving deep into why your saw might be refusing to start and, more importantly, how to fix it. This isn’t just a list of generic troubleshooting steps; I’m sharing the specific techniques and insights I’ve gained from years of hands-on experience. So, grab your tools, and let’s get that saw running!

Stihl MS 250 Won’t Start: 5 Insider Fixes for Quick Revive

Let’s cut to the chase. A non-starting chainsaw is a pain, but it’s often a problem with a relatively simple solution. Here are five of the most common culprits I’ve encountered, along with the steps to diagnose and fix them:

1. Fuel System Issues: The Lifeblood of Your Saw

The fuel system is the heart of your chainsaw. If it’s not pumping correctly, your engine won’t fire.

Understanding the Fuel System

The fuel system of your Stihl MS 250 is a carefully calibrated network that delivers the precise mixture of fuel and air needed for combustion. It includes:

• Fuel Tank: Stores the gasoline and oil mixture.
• Fuel Filter: Prevents debris from entering the fuel line.
• Fuel Line: Carries fuel from the tank to the carburetor.
• Carburetor: Mixes fuel and air in the correct ratio.
• Fuel Pump: (Often integrated into the carburetor) Helps draw fuel from the tank.

The Troubleshooting Process

a. Check the Fuel:

• The Obvious But Essential Step: I cannot stress enough how many times a seemingly complex issue boiled down to stale or contaminated fuel. Chainsaw engines are particularly sensitive to fuel quality.
• My Personal Experience: I once spent an entire afternoon tearing apart a carburetor, only to discover the problem was simply old fuel that had separated and gummed up the system.
• The Fix: Drain the old fuel and replace it with fresh, high-quality gasoline mixed with the correct ratio of two-cycle oil (usually 50:1, but always check your saw’s manual).
• Pro Tip: Always use fuel stabilizer, especially if you don’t use your saw regularly. It prevents fuel from breaking down and causing issues.

b. Fuel Filter Inspection:

• Why It Matters: A clogged fuel filter restricts fuel flow, starving the engine.
• The Inspection: Locate the fuel filter inside the fuel tank (it’s usually attached to the end of the fuel line). Use a small hook or bent wire to pull it out.
• Data Point: A study by the Equipment Service Association found that approximately 30% of small engine starting problems are related to fuel filter issues.
• The Fix: If the filter is dirty or clogged, replace it. Fuel filters are inexpensive and readily available.
• My Insight: I always keep a few spare fuel filters on hand. It’s a small investment that can save you a lot of downtime.

c. Fuel Line Check:

• What to Look For: Inspect the fuel line for cracks, leaks, or kinks. Over time, fuel lines can become brittle and deteriorate.
• The Test: Gently squeeze the fuel line. If it feels hard or brittle, it’s time to replace it.
• My Experience: I once had a tiny crack in a fuel line that was almost invisible. It caused intermittent starting problems that drove me crazy until I finally found it.
• The Fix: Replace any damaged fuel lines. Make sure to use fuel line specifically designed for chainsaws, as it’s resistant to fuel and oil.

d. Carburetor Issues:

• The Heart of the Matter: The carburetor is responsible for mixing fuel and air in the correct ratio for combustion. If it’s clogged or malfunctioning, your saw won’t start.
• Symptoms: If you’ve checked the fuel, filter, and lines and the saw still won’t start, the carburetor is a likely suspect.
• The Fix:
  • Carburetor Cleaner: Try spraying carburetor cleaner into the carburetor intake. This can sometimes dislodge minor clogs.
  • Carburetor Adjustment: The carburetor has adjustment screws that control the fuel-air mixture. However, adjusting these screws without proper knowledge can make the problem worse. I recommend consulting your saw’s manual or a qualified technician before attempting to adjust the carburetor.
  • Carburetor Rebuild: If the carburetor is heavily clogged or damaged, you may need to rebuild it. Carburetor rebuild kits are available and include the necessary parts and instructions.
• My Advice: Carburetor work can be tricky. If you’re not comfortable working on carburetors, it’s best to take your saw to a qualified technician. A botched carburetor rebuild can lead to even bigger problems.

e. Primer Bulb:

• Function: The primer bulb helps draw fuel into the carburetor, especially when the saw is cold.
• The Check: Press the primer bulb several times. You should see fuel flowing through the clear fuel line. If the bulb is cracked or doesn’t fill with fuel, it needs to be replaced.
• My Tip: Sometimes, the primer bulb can become stiff and difficult to press. Warming it up slightly (e.g., with your hands) can sometimes help.

2. Ignition System Problems: Sparking the Flame

The ignition system is responsible for creating the spark that ignites the fuel-air mixture in the cylinder. If there’s no spark, there’s no combustion.

Understanding the Ignition System

The ignition system consists of:

• Spark Plug: Creates the spark that ignites the fuel-air mixture.
• Ignition Coil: Generates the high voltage needed to create the spark.
• Flywheel: Contains magnets that trigger the ignition coil.
• Kill Switch: Grounds the ignition system, stopping the spark and shutting off the engine.

The Troubleshooting Process

a. Spark Plug Inspection:

• Why It’s Crucial: A fouled or damaged spark plug is a common cause of starting problems.
• The Inspection:
  • Remove the spark plug using a spark plug wrench.
  • Inspect the electrode (the part that sticks into the cylinder). It should be clean and dry.
  • Look for cracks, carbon buildup, or oil fouling.
• Data Point: According to NGK Spark Plugs, a leading manufacturer, a fouled spark plug can reduce engine power by up to 30%.
• The Fix:
  • Cleaning: If the electrode is just dirty, you can try cleaning it with a wire brush or spark plug cleaner.
  • Gap Adjustment: Check the spark plug gap (the distance between the electrode and the ground electrode) using a spark plug gap tool. The correct gap for your Stihl MS 250 should be specified in the owner’s manual.
  • Replacement: If the spark plug is cracked, heavily fouled, or has a worn electrode, replace it with a new one.
• My Trick: I always keep a spare spark plug in my toolkit. It’s a quick and easy way to rule out a spark plug issue.

b. Testing for Spark:

• How to Do It:
  • Reattach the spark plug wire to the spark plug.
  • Hold the spark plug against the engine block (make sure the metal part of the spark plug is touching the metal of the engine).
  • Pull the starter rope. You should see a bright blue spark jumping across the spark plug gap.
• No Spark? If you don’t see a spark, the problem lies elsewhere in the ignition system.
• Safety First: Be careful when testing for spark. Avoid touching the spark plug wire or the engine block while pulling the starter rope.

c. Ignition Coil Check:

• What It Does: The ignition coil generates the high voltage needed to create the spark. If it’s faulty, it won’t produce a spark.
• Testing: Testing the ignition coil requires a multimeter. You’ll need to measure the resistance of the primary and secondary windings. Refer to your saw’s service manual for the correct resistance values.
• My Experience: I once had an ignition coil that would work intermittently, making it difficult to diagnose the problem. It would start fine sometimes, and then refuse to start at other times.
• The Fix: If the ignition coil is faulty, it needs to be replaced.

d. Kill Switch Inspection:

• The Simple Solution: Sometimes, the kill switch can become stuck in the “off” position, preventing the engine from starting.
• The Check: Make sure the kill switch is in the “on” position. If it feels loose or damaged, it may need to be replaced.
• My Tip: Disconnect the kill switch wire from the ignition coil. If the saw starts, the kill switch is the problem.

e. Flywheel Key:

• What it Does: The flywheel key is a small metal piece that aligns the flywheel with the crankshaft. If it’s sheared or damaged, the timing will be off, and the saw won’t start.
• The Check: Remove the flywheel and inspect the flywheel key. If it’s damaged, it needs to be replaced.
• My Warning: Replacing the flywheel key can be tricky and requires a special tool to remove the flywheel. If you’re not comfortable doing this, it’s best to take your saw to a qualified technician.

3. Compression Issues: The Power Stroke

Compression is the force that squeezes the fuel-air mixture in the cylinder, making it easier to ignite. If the engine doesn’t have enough compression, it won’t start.

Understanding Compression

Compression is created by the piston moving up and down in the cylinder. The piston rings seal the gap between the piston and the cylinder wall, preventing the fuel-air mixture from escaping.

The Troubleshooting Process

a. Compression Test:

• The Tool: A compression tester is a gauge that measures the pressure inside the cylinder.
• How to Do It:
  • Remove the spark plug.
  • Screw the compression tester into the spark plug hole.
  • Pull the starter rope several times.
  • Read the compression gauge.
• What to Look For: The compression reading should be within the range specified in your saw’s service manual.
• Data Point: A healthy Stihl MS 250 should have a compression reading of at least 120 PSI (pounds per square inch).
• Low Compression? If the compression reading is low, it indicates a problem with the piston, piston rings, or cylinder.

b. Causes of Low Compression:

• Worn Piston Rings: Over time, the piston rings can wear down, allowing the fuel-air mixture to leak past them.
• Scored Cylinder: Scratches or damage to the cylinder wall can also cause low compression.
• Leaky Valves: (Less common in two-stroke engines like the MS 250, but possible) Valves that don’t seat properly can leak compression.
• Blown Head Gasket: (Again, less common in two-stroke engines) A blown head gasket can also cause low compression.

c. The Fix:

• Replacing Piston Rings: Replacing the piston rings is a common repair for low compression. It involves removing the cylinder and piston and installing new piston rings.
• Cylinder Repair or Replacement: If the cylinder is scored or damaged, it may need to be repaired or replaced.
• My Advice: Compression problems can be serious and often require extensive repairs. If you’re not comfortable working on the engine, it’s best to take your saw to a qualified technician.

d. The “Finger Test” (A Quick and Dirty Check):

• How to Do It: Remove the spark plug and put your finger over the spark plug hole. Pull the starter rope. You should feel a strong suction on your finger as the piston moves up and down.
• What it Means: If you don’t feel any suction, it’s a sign of low compression. This isn’t as accurate as a compression test, but it can give you a quick indication of whether there’s a problem.

4. Exhaust System Obstructions: Letting It Breathe

The exhaust system removes the burned gases from the cylinder. If it’s blocked, the engine won’t be able to breathe properly and won’t start.

Understanding the Exhaust System

The exhaust system consists of:

• Exhaust Port: The opening in the cylinder that allows the exhaust gases to escape.
• Muffler: Reduces the noise of the engine.
• Spark Arrestor Screen: Prevents sparks from escaping the muffler and potentially starting a fire.

The Troubleshooting Process

a. Spark Arrestor Screen Inspection:

• Why It’s Important: A clogged spark arrestor screen is a common cause of exhaust system obstructions.
• The Inspection:
  • Locate the spark arrestor screen on the muffler.
  • Remove the screen.
  • Inspect it for carbon buildup.
• The Fix:
  • Cleaning: Clean the screen with a wire brush or carburetor cleaner.
  • Replacement: If the screen is damaged or heavily clogged, replace it.
• My Recommendation: I recommend cleaning the spark arrestor screen regularly, especially if you use your saw frequently.

b. Muffler Inspection:

• What to Look For: Check the muffler for any signs of damage or blockage.
• The Test: Remove the muffler and try starting the saw. If it starts without the muffler, the muffler is likely blocked.
• My Experience: I once had a bird build a nest inside my muffler, completely blocking the exhaust.
• The Fix: Clean out any debris from the muffler. If the muffler is damaged, it may need to be replaced.

c. Exhaust Port Inspection:

• What to Look For: Check the exhaust port for carbon buildup.
• The Cleaning: Use a small scraper or screwdriver to remove any carbon buildup from the exhaust port.
• My Warning: Be careful not to damage the cylinder wall when cleaning the exhaust port.

5. Air Filter Issues: Clean Air In, Power Out

The air filter prevents dirt and debris from entering the engine. A clogged air filter restricts airflow, causing the engine to run poorly or not start at all.

Understanding the Air Filter

The air filter is usually made of foam or paper. It’s located in the air filter housing, which is usually on top of the engine.

The Troubleshooting Process

a. Air Filter Inspection:

• The Visual Check: Remove the air filter and inspect it for dirt and debris.
• The Light Test: Hold the air filter up to the light. If you can’t see light through it, it’s clogged.
• Data Point: A study by the Outdoor Power Equipment Institute (OPEI) found that a clogged air filter can reduce engine power by up to 10%.

b. The Fix:

• Cleaning:
  • Foam Filter: Wash the foam filter with warm soapy water. Rinse it thoroughly and let it dry completely before reinstalling it.
  • Paper Filter: Tap the paper filter gently to remove loose dirt. If it’s heavily clogged, replace it.
• Replacement: Replace the air filter regularly, especially if you use your saw in dusty conditions.
• My Pro Tip: I always keep a spare air filter on hand. It’s a cheap and easy way to keep your saw running smoothly.

Beyond the 5 Fixes: Other Considerations

While the five fixes above address the most common starting problems, there are a few other things to consider:

• Choke: Make sure the choke is engaged when starting a cold engine. The choke restricts airflow, enriching the fuel-air mixture and making it easier to start.
• Flooding: If you’ve pulled the starter rope repeatedly without the engine starting, you may have flooded the engine. To clear a flooded engine, remove the spark plug and pull the starter rope several times. Then, reinstall the spark plug and try starting the engine again.
• Incorrect Starting Procedure: Refer to your saw’s owner’s manual for the correct starting procedure.
• Altitude: At higher altitudes, the air is thinner, and the engine may need to be adjusted to compensate. Consult your saw’s manual or a qualified technician for information on adjusting the carburetor for altitude.
• Temperature: Cold weather can make it more difficult to start a chainsaw. Try warming up the engine slightly before starting it. You can do this by placing the saw in a warm room or using a heat gun to warm the engine block.

Preventative Maintenance: The Key to Longevity

The best way to avoid starting problems is to perform regular preventative maintenance. Here are some tips:

• Use Fresh Fuel: Always use fresh, high-quality gasoline mixed with the correct ratio of two-cycle oil.
• Use Fuel Stabilizer: Add fuel stabilizer to the fuel tank, especially if you don’t use your saw regularly.
• Clean the Air Filter Regularly: Clean the air filter every time you use your saw.
• Clean the Spark Arrestor Screen Regularly: Clean the spark arrestor screen regularly.
• Inspect the Spark Plug Regularly: Inspect the spark plug regularly and replace it as needed.
• Sharpen the Chain Regularly: A sharp chain will make your saw easier to start and use.
• Store Your Saw Properly: Store your saw in a clean, dry place.
• Follow the Manufacturer’s Recommendations: Follow the manufacturer’s recommendations for maintenance and service.

Wood Science and Firewood Preparation: Maximizing Efficiency

Beyond the mechanics of your chainsaw, understanding the science behind wood and firewood preparation can significantly improve your efficiency and the quality of your firewood.

Wood Anatomy and Properties

• Hardwood vs. Softwood: Hardwoods (deciduous trees) generally burn longer and hotter than softwoods (coniferous trees) due to their denser structure. However, softwoods ignite more easily.
• Moisture Content: The moisture content of wood is the percentage of its weight that is water. Freshly cut (“green”) wood can have a moisture content of over 100% (meaning it weighs more than its dry weight).
• Seasoning: Seasoning is the process of drying wood to reduce its moisture content. Properly seasoned firewood should have a moisture content of 20% or less.
• Why Seasoning Matters: Burning unseasoned wood is inefficient, produces more smoke, and can damage your chimney. The energy that should be producing heat is instead used to evaporate the water in the wood.
• Data Point: The U.S. Department of Energy estimates that burning unseasoned wood can reduce heating efficiency by as much as 50%.

Firewood Seasoning Techniques and Safety Considerations

• The 6-12 Month Rule: As a general rule, hardwoods need to be seasoned for at least 12 months, while softwoods can be seasoned in as little as 6 months. However, the actual seasoning time will depend on the type of wood, the climate, and the stacking method.
• Stacking Method: The way you stack your firewood can significantly affect the seasoning time. The ideal stacking method allows for good air circulation around the wood.
• My Preferred Method: I prefer to stack my firewood in single rows, with each row oriented north-south to maximize sun exposure. I also elevate the wood off the ground using pallets or scrap lumber.
• Covering Firewood: Covering firewood can help protect it from rain and snow, but it’s important to leave the sides open to allow for air circulation.
• Testing for Seasoning:
  • Moisture Meter: A moisture meter is the most accurate way to test the moisture content of firewood.
  • The “Thunk” Test: Bang two pieces of wood together. Seasoned wood will produce a sharp “thunk,” while unseasoned wood will produce a dull “thud.”
  • The Smell Test: Seasoned wood will have a dry, woody smell, while unseasoned wood will have a green, sappy smell.

Logging Tool Selection and Maintenance Best Practices

• Beyond the Chainsaw: While the chainsaw is the primary tool for felling and bucking trees, other tools are essential for safe and efficient logging.
• Felling Axe or Maul: Used for driving wedges and felling small trees.
• Wedges: Used to prevent the tree from pinching the saw blade during felling.
• Peavey or Cant Hook: Used for rolling and moving logs.
• Skidding Tongs: Used for dragging logs.
• Personal Protective Equipment (PPE): Always wear appropriate PPE when logging, including a helmet, eye protection, hearing protection, gloves, and chainsaw chaps.
• Tool Maintenance: Keep your tools sharp and in good working condition. A dull axe or chainsaw can be dangerous and inefficient.
• Chainsaw Chain Sharpening: Sharpen your chainsaw chain regularly using a file or a chain grinder.
• Data Point: A sharp chainsaw chain can reduce cutting time by as much as 50%.

Project Planning and Execution: A Case Study

Let’s look at a real-world example of a firewood preparation project:

• The Project: Preparing 10 cords of firewood for the winter.
• The Location: A wooded property in the northeastern United States.
• The Wood: A mix of hardwood (oak, maple, beech) and softwood (pine, fir).
• The Tools: Stihl MS 250 chainsaw, felling axe, wedges, peavey, hydraulic log splitter.
• The Process:
  1. Felling Trees: Select and fell trees, taking care to avoid hazards such as power lines and other trees.
  2. Bucking Logs: Cut the felled trees into manageable lengths (usually 16-18 inches).
  3. Splitting Wood: Split the logs using a hydraulic log splitter.
  4. Stacking Wood: Stack the split wood in single rows, with each row oriented north-south.
  5. Seasoning Wood: Allow the wood to season for at least 12 months.
  6. Moving Wood: Move the seasoned wood to a storage location near the house.
• The Challenges:
  • Weather: Rain and snow can slow down the process.
  • Terrain: Uneven terrain can make it difficult to move logs.
  • Equipment Breakdowns: Chainsaw and log splitter breakdowns can cause delays.
• The Solutions:
  • Working in Good Weather: Plan to work on days with good weather.
  • Using Proper Equipment: Use appropriate equipment for the terrain.
  • Performing Regular Maintenance: Perform regular maintenance on equipment to prevent breakdowns.

Conclusion: Keep Your Saw Sharp and Your Knowledge Sharper

Getting your Stihl MS 250 back up and running can be a rewarding experience. By understanding the common causes of starting problems and following the troubleshooting steps outlined above, you can often fix the problem yourself and save a trip to the repair shop. Remember, preventative maintenance is key to keeping your saw running smoothly for years to come.

But it’s not just about the saw. Understanding wood science, proper seasoning techniques, and the importance of safety will make you a more efficient and responsible wood processor. So, keep your saw sharp, your knowledge sharper, and enjoy the warmth of a well-stocked woodpile this winter!

Now, get out there and put these tips to the test! And remember, safety first! Always wear appropriate PPE and follow safe operating procedures when working with chainsaws and other logging tools. Happy cutting!

Learn more